1. Technical Field
The present invention relates to a printing device and a control method therefor, and relates more particularly to a printing device having a recording paper conveyance mechanism that conveys recording paper, and to a control method for the printing device.
2. Related Art
Printers having a friction-feed recording paper conveyance mechanism that causes a paper feed roller pressed against the recording paper to rotate, and conveys the recording paper only the distance corresponding to how far the feed roller turns, are known from the literature. This type of printer can form lines of printed characters and lines of printed images at a specific printing pitch in the conveyance direction of the recording paper by executing a printing operation by a printhead synchronized to the recording paper conveyance operation of the recording paper conveyance mechanism.
Such printers detect rotation of the paper feed roller or rotation of the motor used as the rotational drive source of the paper feed roller by an encoder, and control how much the recording paper is conveyed based on the detected rotation. However, while rotation of the paper feed roller can track the feed distance of the recording paper with good precision when there is no slipping between the surface of the paper feed roller and the recording paper, when the paper feed roller wears and slipping between the paper and paper feed roller occurs, the paper feed distance is reduced by the amount of slippage.
More particularly, when conveying label paper having labels affixed to the surface of a liner, the shoulders of the labels on the label side of the paper tend to increase the friction load with the printhead while the friction load tends to drop on the back side of the liner that contacts the paper feed roller because the liner is treated to prevent the label adhesive from sticking thereto, and slipping between the label paper and paper feed roller occurs easily. Therefore, when recording paper conveyance is controlled based only on the rotation of the paper feed roller or the motor, the actual feed distance is shorter than the set paper feed distance by the amount of slippage, and paper feed cannot be controlled with good precision. As a result, lines of printed text and lines of printed pixels are formed at a pitch that is narrower than intended in the conveyance direction. Because printing at the correct position is not possible when the printing pitch shifts, print quality drops and the printed information may not be readable, for example.
Japanese Unexamined Patent Appl. Pub. JP-A-H08-230266 describes a printer that prints to the surface of a continuous web having adhesive tape affixed to a release paper liner while conveying the web by a feed roller. This printer measures the printout and calculates slippage (the paper feed deficiency caused by slipping), and based on this calculated slippage sets a pitch correction value for the web being printed on. More specifically, the set print length is compared with the measured length of the printout, and the difference therebetween is stored as correction data. The printer then compensates for this slippage in the next printing operation by adding the number of steps corresponding to the correction data (that is, the slippage) to the drive step count of the drive motor that conveys the tape only the print length.
With the compensation method described in JP-A-H08-230266, the user manually sets and configures the correction data. The calculated correction data is stored in memory disposed to a cassette that holds the tape. As a result, once the correction data is stored in memory, compensation based on this correction data is enabled by simply installing the tape cassette.
However, this method of manually calculating and storing correction data in memory in each tape cassette means that the correction data must be set individually for each tape cassette. In addition, in order to always compensate accurately for slippage, even when slippage changes as a result of conveyance mechanism wear, this correction operation must be performed and the correction data updated frequently. The burden on the user is therefore great.
The correction method taught in JP-A-H08-230266 also adjusts the conveyance distance in the drive step units of the drive motor. This means that the paper feed distance correction unit may be greater than the actual amount of slippage when slippage is slight, and the paper feed distance cannot be accurately corrected. The problem in this case is a drop in print quality. More specifically, barcodes and other objects requiring high precision printing cannot be printed with the required precision, and read errors can result from the printed barcodes.